Origins of resistance drift and spontaneous crystallization in amorphous Ge2Sb2Te5 (GST) alloys have been investigated to solve the failure of long period of set/reset cycles [1-2]. However, there is still no clear observation of failure behavior in the programming volume itself during the repeated switching cycles. Therefore, in this work, sample devices are cleaved and the cross section of the programming volume is thoroughly observed with atomic scale high resolution transmission electron microscopy (HR-TEM) after 103 and 105 cycles for the stable and the unstable endurance test, respectively as shown in Fig. 1. Fig. 2 (a) and (b) show the HR-TEM image and Selected Area Electron Diffraction (SAED) pattern obtained from the GST programming volume. These figures reveal that after the multiple set/reset cycles hexagonal structured GST and meta-stable GeTe (GT) grains are co-existed in the GST film that is dominantly amorphous phase. If the cooling rate of the GST is not fast enough, the GST begins to crystallize into the GT phase. During the cooling cycle, the falling slope of temperature makes the amorphous GST to be crystallized into the hexagonal GST phase. The hexagonal structured GST is a perfect crystalline state and it is so stable that could not be easily changed into the amorphous state even the multiple set/reset cycles. The GT phases are metastable crystalline structure so that the phase transition can be easily occurred. Therefore, these GT grains may be easily recrystallized and make the resistance drift during the set bias. These hexagonal GST grains, although the grain size is very small, are relatively not easily changed into the amorphous phase comparing with the FCC GST, resulting in an intermediate phase transformation during the quenching process of the reset cycle. And, the GT originated from Sb segregation easily changes into amorphous and crystalline GST. These experimental results may explain